This invention relates to electrophotography. More specifically, this invention discloses electrophotographic imaging members having composite photoconductive insulating layers and processes for utilizing said imaging members. 2. 2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored marking material known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface having a polarity of charge opposite to the polarity of charge on the toner particles.
The developed image can then read or permanently affixed to the photoconductor when the imaging layer is not to be used. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g., zinc oxide/ insulating resin binder) where the photoconductive imaging layer is also an integral part of the finished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above, plain paper copying systems, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding charging/imaging sequence will result in a reduction in the maximum charge acceptance of the photoreceptor. This phenonmenon commonly referred to in the art as "fatigue" has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred being preferred because of its superior photosensitivity.
With the disclosure of the efficacy of poly(N-vinylcarbazole) as a photoconductor for electrophotography (U.S. Pat. No. 3,037,861) and the visible light sensitized compositions containing such polymers (U.S. Pat. No. 3,484,237), organic photoconductive materials have become the focus of increasing interest in the development of electrophotographic imaging systems. Organic photoconductive materials, however, do not possess the inherently photosensitivty to compete commercially with inorganic photoconductors such as amorphous selenium. Even where such organic phtoconductors are complexed with activators (as disclosed in the U.S. Pat. No. 3,848,237 the photogeneration of charge carriers is highly field dependent. In an attempt to take advantage of the best properties of both inorganic and organic photoconductors, certain composite photoconductive insulating layers have been disclosed wherein the light absorption function is performed by a relatively thin layer of photoconductive materials which is contiguous with a relatively thick layer of electronically active polymer, U.K. Pat. No. 1,337,228 and Canadian Pat. No. 932,199. In both the composite photoconductor systems disclosed in each of these patents, light is absorbed by the photoconductive layer which generates hole/electron pairs and thereafter injects one species of the photogenerated carriers into the electronically active layer where it is transported either to ground or to the surface bearing the sensitizing charge thereby selectively discharging the imaging member in the light-struck areas. Both of these patents disclose the use of hole and/or electron transport materials in this electronically active insulating layer. In Canadian Pat. No. 932,199 the electronically active insulating layer is disclosed as comprising a solid solution of a non-polymeric active molecule dispersed in a polymeric matrix. Patentee also indicates the equivalent results are obtainable wherein the electronically active insulating layer comprises a polymer having aromatic or heterocyclic electron acceptor moieties as an integral part thereof. No polymeric materials possessing such properties are specifically disclosed.
The mechanism by which such insulating electronically active layers function is believed to be dependent upon the relative steric relationship of the adjacent aromatic and/or heterocyclic groups within the polymer matrix and the relative distance between such groups. Modification of this steric relationship and/or increasing such distance will impair the ability of such electronically active films to effectively transport charge carriers. For example, uniaxial orientation of poly(N-vinylcarbazole) causes spatial constraint of the pendent carbazyl groups of the polymer. Films of poly(N-vinylcarbazole) which have undergone such orientation manifest significant deterioration in their ability to transport charge carriers and thus it is hypothesized that the constraints on the pendent carbazyl groups have impaired the ability of the film to transport charge carriers. It is expected that where similar spatial constraints are introduced chemically; that is, by restraining the freedom of movement of electronically active groups within a polymer, similar impairment of transport properties can be expected. Specific examples of such impairment would be the incorporation of the electronically active groups within the polymer backbone and thus inhibition of their freedom of movement relative to the electronically active groups in adjacent polymer chains.
Accordingly, it is the object of this invention to remove the above as well as related deficiencies in the prior art.
More specifically, it is the principle object of this invention to provide an electrophotographic imaging member having a composite photoconductive insulating film consisting essentially of two separate but contiguous layers.
It is another object of this invention to provide an electrophotographic imaging member having a composite photoconductive insulating film wherein one layer of said film is entirely polymeric and capable of rapid and efficient transport of electrons.
It is yet another object of this invention to provide an electrophotographic imaging process employing one of the above imaging members.